Roll leaf or hot leaf stamping, or roll leaf coating, are relatively well known processes by which a product surface is provided with a trademark, other decorative motif, or protective topcoat. The process provides essentially that a transfer color or design is brought into contact with a product to be coated in the presence of sufficient heat and pressure to assure transfer of the design onto the product surface.
The process involves the use of a particular transfer material generally referred to as a stamping foil or roll leaf. As hereinafter referred to the foil or roll leaf itself normally comprises a composition of several materials involving different metals such as aluminum, gold, silver, chromium, as well as different colors and designs.
A characteristic foil which is used in the hot stamping process will consist primarily of a thin, plastic carrier film, normally polyester or the like. One surface of the carrier film is provided with a sensitive release agent or resin material.
A protective layer or coating is next applied to the heat sensitive release agent. Thereafter one or more layers of the decorative materials are usually applied by vapor deposition. As noted, these layers can consist of a suitable metal such as aluminum, chromium or the like. They could also comprise solid pigments or multi-colored designs.
The decorative layers are next covered with a thickness of a sizing coat comprising a heat sensitive adhesive. This latter thickness will serve to bond the one or more decorative coats to the product surface being treated.
It is understood that many different effects can be produced through the proper combination of colors, metals and the like. In any instance, the decorative layers will be adhered or bonded to the treated surface whereby to give the latter a desired appearance such as that of wood grain, decal or the like.
Operationally, the foil and the product to be decorated are aligned and brought into contact at a transfer station in a machine or appropriate coating fixture. When subject to sufficient release temperature the roll leaf or decorative layers will be separated from the carrier material. Concurrently, the pressure applied at the transfer station, whether by a roll or by a specifically shaped die, will cause the transfer of the roll leaf or decorative layer and its bonding material to the product surface.
A characteristic of the pressure applying surface is such that it will embody a degree of resiliency or yieldability. This feature will account for any irregularities in the surface to be coated. Thus, the pressure applying roll, when the latter is used, must be capable of withstanding the necessary heat as well as the pressure required to perfect the transfer to the product surface.
One type of roller which is found to be acceptable to the purpose is embodied in a cylindrical core to which a silicone rubber coating surface is bonded. To function properly, the roller's outer contact face must be sufficiently thick to maintain the desired degree of resiliency. Depending on the particular application to which the roll is required, this outer layer on the roll can be between 1/32 to 1/2 inch thick.
A further consideration in roll design, however, is the need to raise the contact surface to a temperature level necessary to effect separation of the decorative layer from the carrier material by melting the release agent. The roll is therefore usually provided with both external and internal heating elements normally electrically powered.
Operationally, some of these elements which function as the secondary heat source, must be of sufficient size and capacity to fit within the roll core and yet bring the roll peripheral surface to the required release agent melting temperature. In so doing, however, over a period of time the elevated temperature will eventually lead to the physical deterioration of the rubber, and its eventual separation from the metallic core.
Aside from the deleterious effect of heat on the roll's silicone rubber, the relatively thick layer mandates a greater power requirement. For example, while achieving a desired temperature at the roll surface, the layer itself exhibits a relatively steep temperature gradient. It is therefore necessary, that to maintain a predetermined peripheral temperature, the inner surface of the roll's rubber layer be made considerably hotter than the exposed periphery.
Silicone rubber, although determined to be effective in this thermal transfer process, is also a poor conductor of heat. It is known for example that for each 0.030 inches of silicone rubber thickness, there is a 25.degree. F. temperature drop. Therefore, to maintain a consistent surface temperature at the roll's working area, the heat generating source must be of a capacity as dictated by the thickness of the poor heat conductive rubber layer.
Another prevalent operational defect which is encountered in pressure applying rolls of the typical roll leaf coating apparatus, resides in the rapid deterioration of the roll's journal means. For example, as is most normally employed, needle or roller bearings are mounted to each end of the pressure applying roll to assure a minimal degree of friction as the roll is driven. However, since the rolls, as noted will be heated from within, the end bearings will normally operate at a relatively high temperature.
Not only will prolonged exposure to such an environment and physical climate deteriorate lubricating fluid in the bearings, but it will also initiate strains, particularly in the instance of needle bearings. Further, any such deterioration in the quality of the bearings will be reflected in inaccuracies in the roll leaf transfer operation.
Toward overcoming the above stated problems, particularly with respect to operation of the pressure element, the present invention provides a novel means for supplying sufficient and accurate heat to facilitate transfer of roll leaf layers at higher speeds and productivity rates.
The apparatus presently disclosed for achieving the above includes a flexible, thermal belt which defines a closed loop. A belt drive means causes the belt to be guidably moved through a preset heating and cooling circuit. At least one part of the circuit is provided with a heater element bank or heat source, to bring the belt within a desired temperature range.
The driven belt is roller guided to be brought while in maximum heated condition, into heat exchange engagement with at least a portion of the peripheral surface of the roll or pressure element. While contiguous surfaces of the rotating roll and the belt are in contact, a desired heat flow will take place from the belt directly to the roll surface.
It is therefore an object of the invention to provide an improved roll leaf coating apparatus of the type contemplated. A further object is to provide a thermal belt which is utilized to transfer sufficient heat, whereby to effect separation of a roll leaf or decorative layer from its carrier, and to insure its application to the product surface. Another object is to provide an accurately temperature controlled thermal belt for establishing a proper atmosphere in which to transfer a roll leaf or decorative layer to a product surface. A still further object is to provide a means for effecting a transfer of heat to a pressure roll, whereby to most efficiently utilize the minimal amount of heat available and thereby reduce the power requirements of the overall operation.